In Should You Choose RELAX or JSON Now?, Robert Sayre claims "100% of the data-oriented use cases can be served better and faster by JSON. No 80/20 truism."
I've had a love / hate relationship with JSON over the last year, but I'm currently very positive on it. It has all the basic data types you need, and no more: numbers, strings, boolean, hash tables, arrays, and null. And it's not a document.